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PUBLICATIONS Journal of Geophysical Research: Space Physics RESEARCH ARTICLE 10.1002/2014JA020452 Key Points: • The penetration of IMF into magnetotail is delayed by 1–1.5 h • The delay time strongly depends on the polar convection intensity • The penetration efficiency is higher than that reported previously

Correspondence to: Z. J. Rong, [email protected]

Citation: Rong, Z. J., A. T. Y. Lui, W. X. Wan, Y. Y. Yang, C. Shen, A. A. Petrukovich, Y. C. Zhang, T. L. Zhang, and Y. Wei (2015), Time delay of interplanetary magnetic field penetration into Earth’s magnetotail, J. Geophys. Res. Space Physics, 120, 3406–3414, doi:10.1002/2014JA020452. Received 29 JUL 2014 Accepted 6 APR 2015 Accepted article online 8 APR 2015 Published online 6 MAY 2015

Time delay of interplanetary magnetic field penetration into Earth’s magnetotail Z. J. Rong1,2,3, A. T. Y. Lui4, W. X. Wan1,2, Y. Y. Yang5, C. Shen5, A. A. Petrukovich6, Y. C. Zhang5, T. L. Zhang7, and Y. Wei1,2 1

Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China, 2Beijing National Observatory of Space Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China, 3Swedish Institute of Space Physics, Kiruna, Sweden, 4Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland, USA, 5State Key Laboratory of Space Weather, Center for Space Science and Applied Research, Chinese Academy of Sciences, Beijing, China, 6Space Research Institute, Russian Academy of Sciences, Moscow, Russia, 7 Space Research Institute, Austrian Academy of Sciences, Graz, Austria

Many previous studies have demonstrated that the interplanetary magnetic field (IMF) can control the magnetospheric dynamics. Immediate magnetospheric responses to the external IMF have been assumed for a long time. The specific processes by which IMF penetrates into magnetosphere, however, are actually unclear. Solving this issue will help to accurately interpret the time sequence of magnetospheric activities (e.g., substorm and tail plasmoids) exerted by IMF. With two carefully selected cases, we found that the penetration of IMF into magnetotail is actually delayed by 1–1.5 h, which significantly lags behind the magnetotail response to the solar wind dynamic pressure. The delayed time appears to vary with different auroral convection intensity, which may suggest that IMF penetration in the magnetotail is controlled considerably by the dayside reconnection. Several unfavorable cases demonstrate that the penetration lag time is more clearly identified when storm/substorm activities are not involved.

Abstract

1. Introduction Past studies indicate that the magnetic field By component (geocentric solar magnetospheric (GSM) coordinates) in the Earth’s magnetotail may play an important role in controlling magnetotail dynamics [Rong et al., 2012, and reference therein]. There are multiple sources contributing to the By component in the magnetotail [Petrukovich, 2011]. Among these sources, the penetration of interplanetary magnetic field (IMF) By component into the magnetotail is well known, and the good correlation between IMF By and the tail By component was noted in many previous studies [e.g., Fairfield, 1979; Lui, 1984; Kaymaz et al., 1994; Shen et al., 2008; Cao et al., 2014; Rong et al., 2014; Petrukovich, 2011, and reference therein]. Several possible mechanisms have been proposed to interpret the IMF By penetration [Cowley, 1981; Moses et al., 1985; Hau and Erickson, 1995]. Although the correlation with IMF By component (upstream at 1 AU) had been statistically noticed, no one knows whether or not the tail By component is immediately controlled by the external IMF By, even if the time of solar wind propagation to the downstream is taken into account. To solve this issue explicitly can advance our knowledge about how IMF penetrates and controls the magnetotail dynamics. In one case, McComas et al. [1986] noticed that after 45 min when the IMF had completed an excursion from strong positive IMF By, the By component in plasma sheet still kept the polarity sense of the IMF By. However, this case cannot be seen as the direct evidence for the IMF delay because it did not address the question how to exactly define the delay of IMF By penetration, and no more cases related to this issue are provided afterward. Here with carefully selected cases, the penetration of IMF By into magnetotail with clearly defined time delay is reported for the first time from the direct observations.

2. Event Analysis For this study, several points are noted in selecting the appropriate cases. ©2015. American Geophysical Union. All Rights Reserved.

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1. Since typical IMF strength at 1 AU ranges from 5 to 30 nT, if the IMF can indeed penetrate into Earth’s magnetosphere, it is expected that the magnetotail (x < 10 RE) would be the ideal site to observe the

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Figure 1. A case showing the delay of IMF By penetration on 17 August 2001. Panels from top to bottom show (a) the dynamic pressure of solar wind and (b) the three components of IMF from OMNI data set; (c) the three components of magnetic field measured by C3; (d) the comparison of By component from C3, Geotail, and OMNI data set; (e) the plasma velocity of C3 HIA; (f) the AL index and the PC index on the northern hemisphere; and (g) the symmetric disturbance index. The locations of C3 in GSM are listed in the bottom.

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response clearly and definitely as the typical magnetic field strength in magnetotail lobe, around 20–30 nT, is comparable to IMF. In the magnetotail, Bx and Bz components are very sensitive to the cross-tail current but not the By component. Therefore, concentrating in monitoring the response of By component to IMF By component would not be affected significantly by the cross-tail current variability. It is desirable to have a jump variation of IMF By component (the jump of IMF By should exceed 5 nT, and keeping the polarity and amplitude unvaried significantly for 1 h at least) coincident with the jump of solar wind dynamic pressure (Pdy) for unambiguous timing on the arrival of IMF at the magnetopause. This is because a jump of Pdy usually yields an instant response of magnetotail [e.g., Huttunen et al., 2005], which can be used as the contact time of the external IMF By jump with the magnetotail. The tail-flaring magnetic field could generate the By component which is roughly proportional to the local Bx component [e.g., Fairfield, 1979; Petrukovich, 2011], i.e., By ~ kBx. Nominally, the flaring factor k is positive/negative when spacecraft locates at postmidnight/premidnight, and the magnitude increases toward both flanks. One has to consider the effect carefully when choosing the appropriate events. The OMNI data set with 1 min resolution can provide long-term solar wind parameters (geomagnetic indexes, e.g., AL, PC, and SYM-H, are also merged in OMNI data set), which are time-shifted to the arrival time at the nose of the bow shock [King and Papitashvili, 2005].

With the consideration of the above points, several cases are fortunately found after manually searching the Pdy jump events over the whole 23rd solar cycle (1995–2009) in the OMNI data set with joint observation available in magnetotail. In the following, two “best” cases and several unfavorable cases are shown for the report. GSM coordinates are used throughout the study. 2.1. Case 1 on 17 August 2001 As shown in Figure 1, an evident jump increase of Pdy occurs at ~11:05 from OMNI data set (Figure 1a), which is recorded by the jump increase of geomagnetic magnetic field as indicated by SYM-H index at ~11:03 (Figure 1g). RONG ET AL.


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Coincidently, the IMF By component shows an enhancement from ~5 to ~20 nT accompanying the Pdy jump, whereas the other IMF components do not exhibit the comparable jump magnitude of IMF By (Figure 1b). The Geotail spacecraft [Nishida, 1994], located at the upstream [x = 18.6, y = 21.0, z = 7.0] RE, also records the signature of the Pdy jump at 11:00 (Figure 1d). After the Pdy jump, the decreasing AL index indicates that the westward electrojet in auroral region is enhanced (Figure 1e). The PC index on northern hemisphere, PCN, indicates that the magnetic activity over polar caps increased significantly (Figure 1e) [Troshichev et al., 1988]. Fortunately, when Pdy jump impinges on the magnetosphere, the Cluster satellites [Escoubet et al., 2001] are located at [x = 18.3, y = 4.3, z = 4.3] RE, just in the magnetotail north lobe region. Since four satellites record the similar time series of magnetic field data, only the data of Cluster-3(C3, Samba) are shown here for simplicity. At the time of the vertical black line, a jump of the lobe field Bx component is recorded by C3 at 11:05 due to the magnetotail compression by the jump of Pdy (Figure 1c). After the jump, Bx increases from ~25 nT to ~45 nT, and Bz component increases gradually to ~15 nT. However, although it is probably accompanied by the substorm process as indicated by the AL index, the increased Bz is unlikely the result of tail substorm dipolarization, which is usually manifested by the sudden decrease of Bx and increase of Bz. Here the increase of Bz is likely induced by the compression of Pdy instead [Miyashita et al., 2010; Rong et al., 2014]. The By component does not show any particular variation except for the pulse variation at the moment of Bx jump. During 11:40–11:52, bursty plasma flows are detected from Hot-Ion Analyzer (HIA) measurement [Rème et al., 2001], which induces transient fluctuation of By component and the drop of Bx component (Figure 1e). At the time of the vertical red line, i.e., ~12:00, the By component shows the significant increase then but not prior to that time. In other words, after the jump of Pdy, in the interval 11:05–12:00, magnetotail field has no evidence of any response to IMF until 12:00. The response to IMF lags behind the Pdy jump by nearly 1 h. The time lag can be seen more clearly in Figure 1d, where the comparison of IMF By component from OMNI, Geotail, and the tail By component from C3 is given. We should remind that the observed lobe By component generally contains the contribution from tail-flaring field. The flaring-related By can be roughly estimated as k × Bx, where k is the local flaring coefficient. Nonetheless, due to the temporal variation, the actual scatter of k is rather large (see Figure 2 of Petrukovich [2009]). For Case 1, we estimated the coefficient k before and after Pdy jump. The yielded k ~ 0.64 before Pdy jump (10:00–11:05) and k~0.14 after Pdy jump (11:12–12:00) demonstrate that it is very hard to exactly estimate the flaring effect so as to separate the By component of IMF By penetration. Although the flaring effect cannot be estimated exactly, we can still identify the arrival of penetration easily. In terms of flaring effect, |By| should be proportional to |Bx|. The Bx is stronger (Bx~40 nT) during 11:12–12:00 than that before Pdy jump (Bx~20 nT), but the By component does not show the evident increase during 11:12–12:00. Therefore, the tail flaring should be ignorable in this case. Moreover, after the 12:00 shortly, there are two drops of Bx, we do not observe the drops of By, and then the By component keeps the larger value with same polarity of IMF By even crossing the current sheet to the opposite lobe (after 14:50, not shown here). Therefore, for Case 1, we argue that the influence of flaring effect is not significant; the enhanced By around 12:00 is mainly brought by the IMF By penetration. 2.2. Case 2 on 17 August 2003 Figure 2 shows the other case for the lagged IMF penetration. The signature of Pdy jump was recorded by the geomagnetic SYM-H index at ~14:20 (Figures 2a and 2g). The strength of IMF By starts to increase after the jump of Pdy, up to ~ 20 nT at ~15:12 (Figure 2b). IMF Bz maintains positive with enhanced strength after the Pdy jump, leading to much weaker polar geomagnetic activities as indicated by the AL index (Figure 2e) and PCN index (Figure 2f). Fortunately, at the time of the vertical black line, C3 records the response of tail magnetic field to the Pdy jump at 14:25 at [x = 15.64, y = 4.12, z = 5.31] RE in the tail north lobe region. After the response to the Pdy jump, the Bx component in tail lobe increases significantly as in the previous case shown in Figure 1, whereas the By component has the negligible strength and did not show any particular variation. Not until ~15:54 does the By component show significant variation. Nonetheless, the negative excursion of By occurring at ~15:54 only lasts for 4 min. After ~15:59 (the vertical red line), the tail By shows consistent RONG ET AL.


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Figure 2. (a–g) The other case of IMF By penetration on 17 August 2003. The format is the same with Figure 1.

increase with larger magnitude. In other words, in the interval 14:25–15:59, the tail By component has no evidence of any response to the IMF By until 15:59. The response to IMF lags behind the Pdy jump by nearly 90 min. The time lag can be seen more clearly in Figure 2d, where the comparison of IMF By and the tail By is given. Then, after 15:59, the spacecraft crosses the current sheet to the opposite lobe region keeping the increment of By magnitude. Due to the unavailability of plasma flow, the plasma velocity is not shown in Figure 2. After 16:25, the tail magnetic field showed considerable fluctuations. C3 dips to current sheet center, and Bz became negative several times. The tail magnetic reconnection or the other internal processes may operate to cause the fluctuation of By component [Nakamura et al., 2008]. We evaluate the influence of tail flaring on diagnosing the penetration. The analysis on the interval 14:28–15:52 shows that the average flaring coefficient k is 0.074, which means that the By component from flaring effect can be ignored. Therefore, the increment of By magnitude or the south-north asymmetry of By after 15:59 should be brought directly by the IMF By penetration. During 16:00–16:08, spacecraft is crossing the current sheet. We also evaluate the other possible resources for generating the current sheet By. Based on the study of Petrukovich [2011], apart from IMF By penetration, the warping, twisting of current sheet, as well as the dipole tilt angle can also induce the By component in current sheet: (1) the warping yielded By can be estimated as Bwarp ¼ tanðφwarp ÞBz . The warp angle φwarp is generally y negligible within |y| < 5 RE. For Case 2, y~5 RE and Bz~3 nT for |Bx| < 5 nT; thus, the By from this source should be ignored. (2) The twisting yielded By is related with the IMF By and could be estimated as Btwist ∼ 0:1BIMF y y . Considering that IMF By ~ 20 nT, the average twisting yielded By should be 2 nT, which is much less than that in the current sheet (By~10 nT). (3) Petrukovich [2011] argued that the dipole tilt angle can also yield the By component. However, the maximum By component related with dipole tilt angle is only ±1–2 nT. Therefore, the main source for the strong By component (By~10 nT) in current sheet should be the penetration of IMF By which had arrived at 15:59.

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Figure 3. (a–g) Case of IMF By penetration into magnetotail on 24 July 2004. The format is the same with Figure 1.

3. Unfavorable Events We have also investigated the other events. Some events indeed show the delay of IMF penetration, but the timing of the delay is less clear than the above two cases, while some other events do not show the evident delay penetration of IMF By. The unfavorable reasons for those events are generally complicated. To explore the favorable observation conditions for the IMF penetration, we show several unfavorable cases here by contrast with the two well-identified cases above. 3.1. Case on 24 July 2004 As shown in Figure 3, the Pdy from OMNI data set shows sudden depression at ~12:45, and IMF By gradually increases from ~10 nT to ~20 nT. Actually, as recorded by Geotail at [x = 5.3, y = 27.9, z = 6.9] RE in the upstream solar wind, the IMF By shows jump variation accompanying the sudden Pdy depression. The signature of Pdy depression cannot be identified easily by SYM-H index at this time, because the depression of AL and SYM-H indexes suggests the coincidental occurrence of a substorm onset. As labeled by the vertical black line at 12:48, the depression of Bx recorded by C3 at [x = 12.0, y = 10.8, z = 6.2] RE in magnetotail might be induced by the depression of Pdy. However, the coincidently increased Bz component and sudden depression of AL several minutes later make us believe that the Bx depression should be induced by the substorm dipolarization. Therefore, although the tail By component evidently shows a lag increase at 13:20 as being labeled by the vertical red line after the substorm, we cannot take this case as a favorable event for diagnosing the lag time of penetration. 3.2. Case on 24 August 2005 For the case shown in Figure 4, the Pdy jump yields a corresponding signature of SYM-H index at ~06:13. The IMF By shows a sudden jump from ~10 nT to ~20 nT accompanying the Pdy jump. At the time of the vertical black line, C3 records the response of tail magnetic field to the Pdy jump at 06:17 at [x = 16.6, y = 4.53, z = 0.84] RE. During 06:30–07:10, the tail By shows positive enhancement but exhibiting positive proportional variation with Bx component. Considering that the spacecraft is located at the postmidnight RONG ET AL.


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Figure 4. (a–g) Case of IMF By penetration into magnetotail on 24 August 2005. The format is the same with Figure 1.

region, it is reasonable to believe that the By variation is induced by the tail-flaring effect rather than the IMF By penetration. At ~07:20, a substorm dipolarization occurs, the tail magnetic field shows much fluctuation, and the strength of tail By drops to the lower level. As labeled by the vertical red line at ~08:00 after the substorm onset, the tail By starts to increase gradually and up to ~20 nT after 08:20. If we take ~08:00 as the time responding to the IMF By penetration, the lag time for the penetration would be ~100 min, which is comparable to the time scale as we reported in the case of Figure 2. Nonetheless, in view of the reconfiguration and fluctuation of magnetic field during the substorm depolarization 07:20–08:00, the identified time, ~08:00, for responding IMF By penetration may not be the true penetration time; i.e., the reconfiguration and fluctuation of magnetic field may smear the identification of IMF By penetration, and therefore, we cannot classify this case as a favorable event. 3.3. Case on 19 April 2002 Figure 5 shows another unfavorable event. As labeled by the vertical red line, Geotail records the response of tail magnetic field to the Pdy jump at 08:37 at [x = 29.2, y = 2.5, z = 5.7] RE. Although the IMF By strength shows a jump increase from ~ 10 to ~ 20 nT being coincident with the Pdy jump, no clear tail By enhancement is identified as responding to the IMF By penetration. Note that within the interval 08:53–09:46, the tail By shows minor increase, but we cannot simply attribute it as the IMF By penetration. At least two factors are probably involved for the minor increase: (1) the tail-flaring effect. Before the tail response to Pdy jump, By ~ 5 nT, Bx~17 nT, so that the flaring coefficient k = By/Bx ~ 0.3. During 09:06–09:33, By ~ 10 nT, Bx~40 nT, and a comparable k ~ 0.25 is yielded. On the other hand, during 09:33–09:46, the Bx component is decreased from ~40 nT to ~35 nT and the By magnitude is accordingly decreased to ~5 nT, which fits the characteristic of flaring effect. (2) the internal processes. At 08:53, the magnitude increase of By occurred coincidently to the jump of Bz show. The jump of Bz, followed by the field fluctuation, might be the signatures of some dynamic processes, e.g., dipolarization front [Nakamura et al., 2002]. In other words, the minor increased By during 08:53–09:33 is probably associated with the processes. Meanwhile, the external IMF By keeping ~ 20 nT lasts for 2 h at least, but the minor increased By component is just observed in a much shorter interval, which cannot be the corresponding penetration of external IMF By. RONG ET AL.


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Figure 5. (a–g) Case of IMF By penetration into magnetotail on 19 April 2002. The format is the same with Figure 1, but the tail magnetic field measurement is made by Geotail.

As demonstrated by the SYM-H index, this event is at the recovery phase of a storm, and a substorm should have been occurred during 09:20–10:20 from the AL index. Thus, the failure in identification of the penetration might be associated with the storm/substorm processes. Another similar event that had been checked occurs during 02:00–05:00 on 27 July 2004 (not shown here) when TC-1, Cluster, and Geotail are in the magnetotail. This event is at the main phase of a geomagnetic storm. The external IMF By from OMNI is about ~25 nT after Pdy jump, but no obvious response of tail By to the penetration is detected by the spacecraft.

4. Conclusion and Discussion With two carefully selected cases, we report the direct evidence that the response of magnetotail to the IMF lags behind that of the solar wind dynamic pressure. It is expected that there would be time difference between Pdy and IMF for propagation inside the magnetosheath. However, the time difference is estimated to be only 5–7 min [Zhang et al., 2011]. This lag time can be explained in terms of IMF being “frozen” in the shocked solar wind (magnetosheath) with reduced velocity, while disturbance of Pdy is transported with a faster magnetosonic speed. In our study, the IMF By penetration into magnetotail is nearly 1 h for Case 1 and ~90 min for Case 2. These delays are much longer than the lag time expected in the magnetosheath traversal. The difference in lag time for both cases indicates that the response time of magnetosphere to the IMF is variable. The prominent difference between the two cases examined here is the polar geomagnetic activity as indicated by the AL and PC indexes. Since the PC index can be seen as the proxy for the solar wind electric field applied to the polar cap for driving the polar convection [Troshichev et al., 2000], the larger amplitudes of AL and PC indexes for Case 1 indicate that the convection intensity would be stronger than that in Case 2. Meanwhile, the lag time 1–1.5 h is comparable to the typical time scale for the convection. Thus, it seems that the lag time depends on the convection intensity: the stronger the convection intensity, the shorter the lag time. Since the convection is usually assumed to be driven by the dayside reconnection, it is plausible that the dayside reconnection controls the IMF penetration considerably.

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Our interpretation of the lag time given above is consistent with the penetration mechanisms presented previously. Cowley [1981] predicted that the IMF By can penetrate into magnetosphere via dayside reconnection, which would generate asymmetric magnetospheric effects including opposite senses of azimuthal convection flow in conjugate high-latitude hemispheres. Moses et al. [1985] observed that half of the flow can return in the closed field lines and suggested that the opposite sense flow in closed field lines in conjugate high-latitude hemispheres could tilt the nightside magnetic field lines, resulting in a By with the same polarity of the IMF By. Hau and Erickson [1995] argued theoretically that apart from the shear effect of flow, the By component in plasma sheet could be enhanced due to the compression of earthward convection. This argument, however, cannot be conclusively verified here, because the Cluster locations in both cases when the penetrated By is observed are basically in the lobe regions (Bx > 20 nT). Meanwhile, this mechanism plays a role only in the presence of a finite penetrated By. It is interesting to note for Case 1 that in the interval 12:00–12:13, the By component increased up to ~ 20 nT, which is comparable to the external IMF By, and dropped to the sustained ~10 nT from then on. Similarly, for Case 2, in the interval 15:54–16:13, the recorded By component increased up to ~ 20 nT with strength comparable to IMF By and dropped also from then on. The reason for this transient behavior is unclear. It may be related to the bursty plasma flow (see Figure 1e for Case 1; plasma velocity is unavailable in Case 2) because of the flow compression or shear effect [Hau and Erickson, 1995; Nakamura et al., 2008]. Several unfavorable cases show that the IMF penetration into magnetotail cannot always be identified clearly. The specific reasons are generally diverse and complicated. Anyway, in comparison with the two carefully selected cases, those unfavorable cases suggest that the period without the storm/substorm activities may favor the identification of IMF penetration and the lag time. This is because the dynamic magnetotail current system during storm/substorm period, e.g., field-aligned currents and cross-tail currents, would induce the severe field fluctuation and reconfiguration, which would smear out the identification of IMF penetration. To our knowledge, no one has reported the lag of IMF penetration before. Therefore, many studies published before about the IMF penetration on magnetotail [e.g., Cowley, 1981] may need to be reexamined with the consideration of time delay in IMF penetration. For example, previous statistical surveys found that the penetration rate, i.e., the ratio of lobe By to IMF By in lobes, is 9–15% [e.g., Tsurutani et al., 1984; Sergeev, 1987; Kaymaz et al., 1994]. Considering the delay time, the penetration rates in the two cases are ~50%, much higher than the previous estimations.

Acknowledgments The authors are thankful to the ESA Cluster Active Archive (http://caa.estec. esa.int/caa/home.xml), DARTS at ISAS JAXA (http://darts.isas.jaxa.jp/stp/geotail), and the GSFC/SPDF OMNIWeb interface (ftp://cdaweb.gsfc.nasa.gov/ pub/data/omni/) for providing the Cluster FGM data, Geotail MGF data, and OMNI data, respectively. This work is supported by the Chinese State Scholarship Fund (201304910018), Chinese Academy of Sciences (KZZDEW-01-2), National Key Basic Research Program of China (2011CB811404 and 2011CB811405), and the National Natural Science Foundation of China (41104114, 41374180, 41321003, 40974101, 41131066, and 41231066). Z.J. Rong would like to acknowledge the hospitality of IRF, Kiruna, Sweden. Larry Kepko thanks the reviewers for their assistance in evaluating this paper.

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Furthermore, the identification of this delay may offer a key clue to interpret the time sequence of magnetospheric processes exerted by IMF. For example, the lag time of IMF, 1–1.5 h, we report here is comparable to the typical time scale of magnetotail energy storage for substorm when IMF is southward [e.g., Baker et al., 1985]. Recently, some studies noticed that the response time of energetic electron and proton flux in plasma sheet to southward IMF Bz is between 40 and 100 min [Luo et al., 2011; Cao et al., 2013], which is also comparable to the lag time of IMF penetration we reported here. Thus, the tail substorm energy storage and the response of energetic particles flux in plasma sheet to southward IMF Bz are probably closely related with the delay of IMF penetration, for which the length of delay determined by the convection intensity. Additionally, previous theory argued that the reduction of solar wind electric field imparted to magnetosphere, manifested by the IMF northward turning or reduction of IMF By, could trigger a substorm [Lyons, 1995]. Therefore, to check the validity of this theory, one has to take into account the delay in IMF penetration, which is ignored before.

References Baker, D. N., T. A. Fritz, R. L. McPherron, D. H. Fairfield, Y. Kamide, and W. Baumjohann (1985), Magnetotail energy storage and release during the CDAW 6 substorm analysis intervals, J. Geophys. Res., 90(A2), 1205–1216, doi:10.1029/JA090iA02p01205. Cao, J., A. Duan, H. Reme, and I. Dandouras (2013), Relations of the energetic proton fluxes in the central plasma sheet with solar wind and geomagnetic activities, J. Geophys. Res. Space Physics, 118, 7226–7236, doi:10.1002/2013JA019289. Cao, J. B., A. Duan, M. Dunlop, X. Wei, and C. Cai (2014), Dependence of IMF By penetration into the neutral sheet on IMF Bz and geomagnetic activity, J. Geophys. Res. Space Physics, 119, 5279–5285, doi:10.1002/2014JA019827. Cowley, S. W. H. (1981), Magnetospheric asymmetries associated with the y component of the IMF, Planet. Space Sci., 29, 79–96. Escoubet, C. P., M. Fehringer, and M. Goldstein (2001), The Cluster mission, Ann. Geophys., 19, 1197–1200. Fairfield, D. H. (1979), On the average configuration of the geomagnetic tail, J. Geophys. Res., 84, 1950–1958, doi:10.1029/JA084iA05p01950.


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Hau, L.-N., and G. M. Erickson (1995), Penetration of the interplanetary magnetic field By into Earth’s plasma sheet, J. Geophys. Res., 100(A11), 21,745–21,751, doi:10.1029/95JA01935. Huttunen, K. E. J., J. Slavin, M. Collier, H. E. J. Koskinen, A. Szabo, E. Tanskanen, A. Balogh, E. Lucek, and H. RÃ me (2005), Cluster observations of sudden impulses in the magnetotail caused by interplanetary shocks and pressure increases, Ann. Geophys., 23(2), 609–624. Kaymaz, Z., et al. (1994), Interplanetary magnetic field control of magnetotail magnetic field geometry: IMP 8 observations, J. Geophys. Res., 99(A6), 11,113–11,126, doi:10.1029/94JA00300. King, J. H., and N. E. Papitashvili (2005), Solar wind spatial scales in and comparisons of hourly Wind and ACE plasma and magnetic field data, J. Geophys. Res., 110, A02104, doi:10.1029/2004JA010649. Lui, A. T. Y. (1984), Characteristics of the cross-tail current in the Earth’s magnetosphere, in Magnetosphere Currents, Geophys. Monogr. Ser., vol. 28, edited by T. A. Potemra, pp. 158–170, AGU, Washington, D. C., doi:10.1029/GM028p0158. Luo, B., W. Tu, X. Li, J. Gong, S. Liu, E. Burin des Roziers, and D. N. Baker (2011), On energetic electrons (>38 keV) in the central plasma sheet: Data analysis and modeling, J. Geophys. Res., 116, A09220, doi:10.1029/2011JA016562. Lyons, L. R. (1995), A new theory for magnetospheric substorms, J. Geophys. Res., 100(A10), 19,069–19,081, doi:10.1029/95JA01344. McComas, C., T. Russell, R. C. Elphic, and S. J. Bame (1986), The near-Earth cross-tail current sheet: Detailed ISEE 1 and 2 case studies, J. Geophys. Res., 91, 4287–4301, doi:10.1029/JA091iA04p04287. Miyashita, Y., et al. (2010), Plasma sheet changes caused by sudden enhancements of the solar wind pressure, J. Geophys. Res., 115, A05214, doi:10.1029/2009JA014617. Moses, J. J., N. U. Crooker, D. J. Gorney, and G. L. Siscoe (1985), High latitude convection on open and closed field lines for large IMF By, J. Geophys. Res., 90, 11,078–11,082, doi:10.1029/JA090iA11p11078. Nakamura, R., et al. (2002), Motion of the dipolarization front during a flow burst event observed by Cluster, Geophys. Res. Lett., 29(20), 1942, doi:10.1029/2002GL015763. Nakamura, R., et al. (2008), Cluster observations of an ion-scale current sheet in the magnetotail under the presence of a guide field, J. Geophys. Res., 113, A07S16, doi:10.1029/2007JA012760. Nishida, A. (1994), The Geotail mission, Geophys. Res. Lett., 21, 2871–2873, doi:10.1029/94GL01223. Petrukovich, A. A. (2009), Dipole tilt effects in plasma sheet By: Statistical model and extreme values, Ann. Geophys., 27, 1343–1352. Petrukovich, A. A. (2011), Origins of plasma sheet By, J. Geophys. Res., 116, A07217, doi:10.1029/2010JA016386. Rème, H., et al. (2001), First multispacecraft ion measurements in and near the Earth’s magnetosphere with the identical Cluster Ion Spectrometry (CIS) experiment, Ann. Geophys., 19, 1303–1354. Rong, Z. J., et al. (2012), Profile of strong magnetic field By component in magnetotail current sheets, J. Geophys. Res., 117, A06216, doi:10.1029/2011JA017402. Rong, Z. J., et al. (2014), Radial distribution of magnetic field in Earth magnetotail current sheet, Planet. Space Sci., 103, 273–285. Sergeev, V. A. (1987), Penetration of the By component of the interplanetary magnetic field into the tail of the magnetosphere, Geomagn. Aeron., 27, 612–615. Shen, C., et al. (2008), Flattened current sheet and its evolution in substorms, J. Geophys. Res., 113, A07S21, doi:10.1029/2007JA012812. Troshichev, O. A., V. G. Andrezen, S. Vennerstrom, and E. FriisChristensen (1988), Magnetic activity in the polar cap—A new index, Planet. Space Sci., 36, 1095–1102. Troshichev, O. A., R. Y. Lukianova, V. O. Papitashvili, F. J. Rich, and O. Rasmussen (2000), Polar cap index (PC) as a proxy for ionospheric electric field in the near-pole region, Geophys. Res. Lett., 27, 3809–3812, doi:10.1029/2000GL003756. Tsurutani, B. T., D. E. Jones, R. P. Lepping, E. J. Smith, and D. G. Sibeck (1984), The relationship between the IMF By and distant tail (150–238 RE) lobe and plasma sheet By fields, Geophys. Res. Lett., 11, 1082–1085, doi:10.1029/GL011i010p01082. Zhang, Y. C., C. Shen, Z. X. Liu, Z. Y. Pu, I. Dandouras, A. Marchaudon, C. M. Carr, and E. Lucek (2011), Magnetopause response to variations in the solar wind: Conjunction observations between Cluster, TC-1, and SuperDARN, J. Geophys. Res., 116, A08209, doi:10.1029/2011JA016462.

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